The present invention relates, in general, to a pallet container.
An example of a pallet container of a type involved here has a thin-walled inner container of thermoplastic material for storage and transport of liquid or free-flowing goods. The plastic container is closely surrounded by a lattice tube frame as support jacket, and rests on a bottom pallet to which the support jacket is fixedly secured. The lattice tube frame includes vertical and horizontal tubular rods which are welded to one another at intersecting areas.
Pallet containers are used for the storage and transport of liquid or free-flowing goods. During transport of filled pallet containers—in particular with contents of high specific weight (e.g. above 1.6 g/cm3)—on poor roads with trucks with firm suspension, during transport on railway or ships, the lattice rod frame is exposed to significant stress as a result of surge forces of the goods. These dynamic transport loads generate significant continuously changing bending stress and torsion stress in the lattice tube frame, ultimately leading to fatigue cracks and resultant rod facture when exposed over respectively long periods.
Lattice tube frames with uniformly continuous lattice tube profile, are known, e.g., in European Pat. Appl. No. EP 0 755 863-A, German utility model no. DE 297 19 830-A, or U.S. Pat. No. 6,244,453 B1. As a consequence of oscillating surge pressure of the liquid content that is caused by fluctuating bending stress during transport, known lattice tube frames fracture in a relatively very short period in the tension zone of the tubular lattice rods. Rod fracture takes place predominantly in proximity of the welded intersections of the tubular lattice rods.
Those lattice tube frames with welded round rods, e.g. disclosed in European Pat. Appl. No. EP 0 734 967 B1, and with significantly reduced tube cross sectional height in the area of the intersections (no continuous tubular profile, dents or reduced tube cross sectional height of same depth) suffer the critical drawback that significant stress peaks are encountered in these areas of reduced tube cross section to thereby form break zones or buckling zones, e.g. during drop tests, when exposed to fluctuating bending stress as a result of transport loads, and during hydraulic internal pressure test. The rod areas between the intersections are much too rigid and stiff when exposed to any dynamic loads and they are unable to absorb deformations which occur only in the intersection area with the decreased tube cross sections. In addition, further quality deterioration or relief areas are necessarily provided in all horizontal and vertical lattice rods at all welding locations, e.g. in afore-mentioned European Pat. Appl. No. EP 0 734 967 B1, to protect them from tearing open/detachment during fluctuating bending stress as a result of transport loads. However, it is considered highly disadvantageous that the weakest tube cross sections are arranged in immediate proximity of the welding spots of the intersecting lattice rods so that the deformation changes continuously directly adjacent to the welding spots. As a consequence, the welding spots are overly stressed and tend to tear off. When it comes to design, the welding expert is aware not to weld dynamically stressed components in those regions that are exposed to the greatest dynamic deformation.
International PCT publication nos. WO 01/89954-A and WO 01/89955-A disclose a pallet container with a trapezoidal tube profile of the lattice rods, wherein the vertical and/or horizontal tubular rods have each a dimple laterally adjacent to an intersection. These partial dimples serve as “bending hinge” and decrease the resistance moment against bending. It has been shown that these limited dimples lead to appreciable longer service life but are unable to completely eliminate a rod fracture when an area is exposed to concentrated stress peaks over a longer period.
Lattice rod frames known to date with uniformly continuous lattice tube profile have all the drawback that the horizontal and vertical tubular lattice rods are generally too rigid and torsionally stiff along their entire length when exposed to fluctuating bending stress; As a consequence, fatigue cracks and rod fracture are encountered already after a comparably short time under stress, in particular in proximity of the welded intersections of the tubular lattice rods.
Known lattice tube frames of welded rounded tubes with reduced tube cross section at the intersections and additional partial lateral relief zones have the following drawbacks:                The height of the reduced tube cross sections must be the same for all welded intersections, it should not be suited to different fluctuating bending stress.        The round tubes with circular cross section next to the intersections welded in dents are very rigid, they do not deform when exposed to fluctuating bending stress.        The round tubes adjacent to the welded intersections are furthermore very torsionally stiff, they do not deform when exposed to torsional stress. The horizontal lattice profile rods are twisted by radial movements of the vertical rods with which they are welded, when exposed to fluctuating bending stress. As a consequence, added tension stress and pressure loads act upon the welding spots.        All loads or stress during transport such as, e.g., pressure stress, tension stress, torsional stress, can be absorbed solely by the locally limited partial dimples (desired buckling zones or fracture zones) directly adjacent the intersections.        
It would therefore be desirable and advantageous to provide an improved pallet container with a lattice tube frame of welded tubular rods, to obviate prior art shortcomings so as to be resistant to fatigue cracks and rod fracture over a long period, while taking into account the stacking load of a loaded stacked pallet container (double stacking) besides the normal transport stress of back and forth sloshing liquid content.